In space applications it is necessary to be able to orientate certain components of a satellite or spacecraft, such as an antenna, a boom, etc., in a predetermined direction in order, for example, to point them at a fixed star, to maintain the aim at a point on the surface of the Earth, or to scan a particular region on the surface of the Earth or of some celestial body. These positionings require frequent corrections to compensate for the inevitable drift of direction, and these multiple corrections involve a large number of cycles of micro-rotation for these vehicular appendages and for their mountings. This phase is known as fine pointing.
Because of the irreversible nature of the insertion into orbit of a satellite, the design of the spacecraft has to build in a very long life, measured in number of cycles, for the mountings, in order to ensure their ability to withstand this enormous number of stresses. For example, the life may be greater than 600,000 cycles. Other requirements of the mountings are great mechanical strength (to withstand external forces and bending moments exerted on the mounted shaft), high transverse stiffness, high-precision zero-tolerance guidance, an amplitude of rotation greater than 1° in both directions, and, a low resistive torque.
It is known practice to construct a mounting for a rotary shaft with a pivot having crossed flexible webs, notably those known by the name “Bendix Pivot”. This type of pivot is not a through pivot. It is therefore necessary to cantilever-mount the mounted shaft, which means large stresses in the webs of the pivot and large stresses being transmitted to the supporting frame, even with a double-pivot mounting. This type of pivot therefore offers limited mechanical strength and transverse stiffness, and does not satisfactorily meet all the requirements of a fine-pointing device.
It is also known practice to support a shaft with a mounting comprising two ball-bearing pivots. Such a device is subject to wear and therefore has a limited life. It also therefore does not satisfactorily meet all the requirements for a fine-pointing mounting design for a space application.